Application of an electric field to a gas, generally under vacuum, to form a plasma of gas ions is a known technique used in polymerization of monomers. This technique has been described, for example, in Luster U.S. Pat. No. 2,257,177. In general, the art teaches continuous contact of the polymerizing monomer in the vapor phase with the gas plasma until substantial completion of the polymerization in a film on a substrate. In addition to being energy intensive, this technique tends to form a cross-linked product which is solvent insoluble and so not suitable for many applications. Specifically, in Tobin U.S. Pat. No. 3,287,242, the degree of completion of polymerization contact with an electrical discharge is illustrated in the examples to be 45 to 60% while no further polymerization is contemplated as the polymer is immediately thereafter recovered. Also, the product is stated to be highly cross-linked.
Due to the high cross-linking associated with plasma polymerization, that technique is generally employed only for the purpose of forming thin films. The state of the art has taught that activation is confined to a region near the surface of the monomer at which links and cross-links form while the monomer in bulk below the surface is not polymerized. See, e.g., col. 1 of Cormia et al U.S. Pat. No. 4,013,532.
One modification of the film forming techniques in which the monomer is polymerized directly from the gas state is described in Knox et al U.S. Pat. No. 3,475,307. There, the substrate is cooled to condense a thin layer of liquid monomer on the substrate in order to increase the polymerization rate. However, that patent teaches one should avoid condensing "too much" of the monomer on the surface because otherwise the incoming activated molecules from the gas phase would not reach the monomer removed from the gas liquid interface which is stated to cause a coating of little adherence (col. 10, lines 54-60). The order of magnitude of condensed monomer prior to polymerization is indicated as being few molecules thickness (col. 4, lines 1-4).
Another process employing an electric current for polymerization is described in Woodman U.S. Pat. No. 2,632,729. There, an aqueous emulsion of a water-insoluble monomer and containing electrolyte is polymerized by conducting electricity through the emulsion using the electrolyte. This reference states that a gas discharge in the gas phase (i.e., one that would create an ionized gas plasma) is to be avoided. Instead, current is to be directed between the liquid and the gas, preferably with the negative electrode immersed in the aqueous dispersion. (Col. 5, lines 5-26). The reference further indicates without detail that the range of molecular weights of the polymeric materials could be controlled by propagation of the polymerization apart from the zone of discharge to vary the length of time during which the volume of dispersed materials is exposed to the discharge. (Col. 6, lines 9-24). There are a number of disadvantages to the Woodman process including the indicated requirement for extraneous electrolyte and emulsifier which are difficult to remove from the final polymer. Furthermore, the system is limited to monomers insoluble in water which are capable of being emulsified therewith.